1. Field of the Invention
The present invention relates to a reflective optical scale for an encoder and a reflective optical encoder.
2. Description of the Related Art
A reflective optical encoder used to obtain position information includes a reflective optical scale, a light source such as an LED which irradiates this scale with light, and a light detector which detects reflected light from this scale. On the reflective optical scale, reflecting regions and non-reflecting regions are alternately arranged, and the reflectance of light on the reflecting region is higher than that of light on the non-reflecting region. Thus, reflected light from the scale generates a fringe-like image (interference fringes) on the light detector arranged to oppose this scale. The light detector detects a phase shift of the fringes caused when the position of the scale moves in the length measurement direction. The reflective optical encoder processes displacement information of the position of the scale according to the detected phase shift of the interference fringes, thus obtaining position information.
One of methods of enhancing detection accuracy of the fringes using the reflected light from the scale is to increase the contrast (density pattern) of the fringes. Thus, incorrect detection of the light detector can be prevented, and the detection accuracy can be improved. Japanese Patent Laid-Open No. 2004-20283 discloses a scale which has a silicon substrate and convex portions periodically formed on the silicon substrate, and in which each convex portion has a silicon oxide film and a monocrystalline silicon layer formed on the silicon oxide film. According to Japanese Patent Laid-Open No. 2004-20283, fringes formed by light reflected by concave portions and the convex portions can be detected. Also, Japanese Patent Laid-Open No. 2008-170286 discloses a scale in which metal layers (reflecting regions) and black underlying layers (non-reflecting regions) are arranged in a grid pattern, and are covered by an antireflection film having an intermediate refractive index of light between those of these films and layers. According to Japanese Patent Laid-Open No. 2008-170286, this antireflection film does not provide any antireflection function of light for the reflecting regions, and absorbs light for the non-reflecting regions, thus increasing the contrast of fringes generated by reflected light. Furthermore, Japanese Patent Laid-Open No. 2004-28862 discloses a scale in which reflective gratings are formed on reflecting regions of the reflecting regions and non-reflecting regions on an antireflection film formed on a transparent substrate. According to Japanese Patent Laid-Open No. 2004-28862, a reflected light amount from portions other than the reflective gratings of a light-receiving amount of a detector, especially, an unnecessary light amount reflected from the transparent substrate is reduced, thus allowing a detection signal to be generated with high accuracy.